In mobile communication, transmission is performed by using a digital modulation to impress data on carrier waves. There exists a large variety of standardised air interfaces, such as the Global System for Mobile Communication (GSM) standard specified by the European Telecommunication Standard Institute (ETSI) using time division multiple access (TDMA), while in the U.S. a number of interim standards such as IS-54 and IS-136 are defined.
The growing number of mobile terminal devices such as mobile phones or communicators yields a bigger demand for voice and data channels within the system of mobile communication. The increasing interference between neighbouring base stations and interference with other devices using radio links decreases the carrier to interference (C/I) or carrier to noise (C/N) ratio. Thus, the quality of mobile phone channels is highly fluctuating, so that transmission of erroneous information or information that is being corrupted during transmission is a very common phenomenon. The quota of erroneous units can be as high as 1% up to 10%.
To handle the problem of large error rates, usually a channel coding (convolutional coding can also be used other than block coding) is used to introduce a redundancy to the data stream, which allows for a recognition and a possible correction of erroneous units transmitted. The definition of such a channel coding is known to every person skilled in the art, and thus a more precise discussion is omitted from the following specification. The reliability of the channel coding is defined by the number of data bits per coded bits. The higher the number of data bits per code bits, the less redundancy is introduced to the data stream and the less encoded information unit is secured against corruption during transmission. If an erroneous unit is received, a retransmission of the (in type II hybrid ARQ, the retransmitted data can be not the same as the first transmission) data is necessary. Accordingly, the basic ARQ scheme provides error detection and retransmission capabilities. If an information unit is found to be corrupted during transmission, it is discarded and a retransmission of the same information unit is requested.
On the other hand a fast transmission of data other than voice channels, such as in the general packet radio service (GPRS) standard or in the upcoming universal mobile telecommunication system (UMTS) standard, is essential due to a large amount of transferred data and a limited transmission rate. The transmission rate is determined among other factors by the reliability of the transferred data. The higher the number of coded bits the poorer the transmission rate. Yet, it is important to select from a large number of data bits per coded bits ν corresponding to the quality of the transmission link to ensure a fast and reliable transmission.
One commonly used scheme is the type II hybrid ARQ. In the type II hybrid ARQ scheme, information is first sent with high (code rating ⅔ as “higher” than ½.) code rating, i.e. little redundancy, and if one erroneous unit is requested to be retransmitted, additional code bits will be added for the following transmission. This enhances the efficiency of the transmission. First, a rather poor coding is used and only if transmission fails due to erroneous units, a more reliable block code is used.
For example, two different types of type II hybrid ARQ schemes can be used. A set of consequent code ratings defined by the number of data bits per code bits ν increasing as 1→½→⅓ is used. Another one is using a finer code rating granularity. This scheme yields a code rating of ν: 1→⅔→½→⅖→⅓. It is appreciated by a person skilled in the art, that a fine code rating granularity results in a higher link throughput but in higher normalised packet delays as well.